In the past, 40-inch (″) class TVs were prevalent, but many consumers now purchase 50″ class TVs and even 60″ class TVs. While the size competition was over, resolution competition has begun. Even a year ago, full high definition (FHD) class TVs belonged to an advanced model, but ultra HD (UHD) TVs now have been rapidly spread on the market.
Recently, the display market has evolved from large-area, high-resolution competition to color competition. For this reason, competition for the production of a display with excellent color has recently emerged.
A liquid crystal display (LCD) displays an image by using optical properties of liquid crystals, wherein, since a liquid crystal panel displaying the image is a non-emissive device or not self-luminous, the liquid crystal display has a structure comprising the liquid crystal panel as well as a back-light unit which is disposed on a rear surface of the liquid crystal panel to provide light to the liquid crystal panel.
Since the liquid crystal display does not entirely transmit light provided by the back-light unit, brightness of the panel is very important. Various films have been developed and used to improve the brightness of the panel, and a typical example of the film comprises a reflective polarizing film.
The reflective polarizing film is typically in the form of a multilayer film in which a layer having a low refractive index and a layer having a high refractive index are alternatingly stacked, and the multilayer film causes optical interference in which light with a specific wavelength is selectively reflected and/or transmitted by interlayer interference. It is known that, when tens to hundreds or more of layers are sufficiently stacked as described above and a thickness of the adjacent layer is gradually changed, reflection and/or polarization characteristics in a desired wavelength range may be improved (see Korean Patent Application Laid-open Publication No. 1997-0700585 and International Patent Application WO 1995-017303).
However, under the circumstances that a picture quality standard has been changed for UHD broadcast, the application of the reflective polarizing film may provide an improvement in terms of brightness, but there is a need for a separate solution in terms of color gamut. Particularly, there is a need to develop a new technique which may dramatically improve a color gamut to a UHD broadcast standard, as a new standard level, while minimizing brightness loss.